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VBA - advanced videos | Excel VBA Part 20a - Application Events
Posted by Andrew Gould on 26 November 2015
In an earlier video in the series we've seen how to write event-handlers for workbooks and worksheets; this video teaches you how to access events of the Excel application itself! You'll learn the significance of the WithEvents keyword, a couple of the basic application events such as the new workbook event, and how to use a basic class module to keep your application event handlers neatly organised.
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I am trying to learn the mertis of creating your own events.
Can you please explain the use of the following code?
This is in Sheet1:
Option Explicit
Dim WithEvents SomeClass As Class1
Private Sub SomeClass_ShowMessage(Message As String)
MsgBox Message
End Sub
Private Sub Worksheet_Change(ByVal Target As Range)
Set SomeClass = New Class1
Call SomeClass.SomeMethod
End Sub
This is in Class1:
Option Explicit
Public Event ShowMessage(Message As String)
Sub SomeMethod()
RaiseEvent ShowMessage("Hi")
End Sub
My (limited!) understanding of this code is that when something changes on Sheet1, a messagebox displays the message "Hi".
But why go to all that length using a class module, as well as Raise Event, Event and WithEvents when you could simply write this in Sheet1:
Private Sub Worksheet_Change(ByVal Target As Range)
MsgBox "Hi!"
End Sub
Thanks
The short answer is that you wouldn't! The example you've given is one to demonstrate the basic principles of custom events. In reality you'd be writing custom events which are meaningful in the context of the class in which they're defined. A common, generic approach is to raise custom events both before and after something has happened in a class. What that something is will be entirely dependent on what you're designing your class to do.
http://cpearson.com/excel/Events.aspx - scroll to the end for a short section on custom events.
https://rubberduckvba.wordpress.com/2019/03/27/everything-you-ever-wanted-to-know-about-events/ - this contains a link to a Battleships game which uses lots of advanced VBA techniques
The following is code form your video:
' ThisWorkbook
Option Explicit
Private xlApp As EventApp
Private Sub Workbook_Open()
Set xlApp = New EventApp
End Sub
' Class Module called EventApp
Option Explicit
Private WithEvents xlApp As Application
Private Sub Class_Initialize()
Set xlApp = Application
End Sub
Private Sub xlApp_NewWorkbook(ByVal Wb As Workbook)
Wb.Worksheets("Sheet1").Range("A1").Value = "Created on " & Date
Wb.Worksheets("Sheet1").Range("A2").Value = "Created by " & Environ("UserName")
End Sub
I have seen this from a book:
' Class Module called EventApp
Public WithEvents xlApp As Application
Private Sub xlApp_NewWorkbook(ByVal Wb As Workbook)
Wb.Worksheets("Sheet1").Range("A1").Value = "Created on " & Date
Wb.Worksheets("Sheet1").Range("A2").Value = "Created by " & Environ("UserName")
End Sub
' Standard Module
Public myAppEvent As New EventApp
Sub TrapAppEvent()
Set myAppEvent.xlApp = Application
End Sub
Both versions work but there are some differences.
1. Your variables are declared Private whereas the other version is declared Public, eg Private xlApp As EventApp / Private WithEvents xlApp As Application
The other version declares: Public WithEvents xlApp As Application / Public myAppEvent As New EventApp
2. The other version contains this line:
Set myAppEvent.xlApp = Application
but yours doesn't.
Can you please explain the significance of these two points.
Thanks
Hi there,
1. Variables declared as Private are accessible only within the module in which they are declared. Variables declared as Public can be referenced from any module in the same project. I declared xlApp as Private in both the class and normal modules so that the variable was specific to each module.
2. The same line appears in my code in the Initialize event of the class module - the Initialize event is triggered when a new instance of the class is created.
I hope that helps!